For an insulation layer of a circuit board, a known technique uses a material in which a cloth is contained in resin. The cloth is made of glass fiber, for example. Such a material insulates conductors, such as lines formed in the circuit board, from each other; and gives the circuit board a sufficient stiffness.
Some circuit boards have a stripline structure in which conductor layers are formed above and below a signal line for signal transmission, via insulation layers. Other circuit boards have a microstripline structure in which a signal line is formed over an insulation layer and a conductor layer is formed under the insulation layer.
See, for example, Japanese Laid-open Patent Publication No. 2014-130860.
In the circuit boards that have the stripline structure or the microstripline structure, dielectric constant of an insulation layer may affect the signal transmission speed of the signal line.
In a circuit board in which a material containing a glass cloth or the like is used for an insulation layer, for example, dielectric-constant distribution periodic due to a yarn pattern or a weave pattern of the cloth may occur in the insulation layer. In such a circuit board having the dielectric-constant distribution of the insulation layer, difference (variations) in signal transmission speed of signal lines may occur, depending on the positional relationship between the signal lines and the dielectric-constant distribution. Such a positional relationship is difficult to control in the manufacturing of the circuit board.
When the difference in signal transmission speed of the signal lines of the circuit board occurs, difference in signal arrival time to a certain circuit may occur in an electronic device which uses the circuit board. This may cause the electronic device to fail to carry out a desired operation.